During the production of semiconductor devices, leadframes are traditionally used as a cost-effective way to mount and process a plurality of semiconductor dice or chips concurrently. Each leadframe typically has a plurality of die pads for mounting the said chips. The leadframe also acts as a means to electrically connect the chip to external devices via leads of the leadframe. Bonding wires are connected to electrical contacts found on the chip and said leads of the leadframe in a process known as wire bonding. The wires usually comprise gold, aluminum or copper material.
After the chips are mounted onto the leadframe and the bonding wire connections are made between the chips and the leadframe, each chip has to be protected from the environment by encapsulating it with a plastic molding compound, such as epoxy molding compound (“EMC”). Each encapsulated chip constitutes a semiconductor package. The multiple semiconductor packages are then diced or singulated to form individual semiconductor devices.
It is important that adhesion between the leadframe material and the molding compound is strong. In the case of copper-based leadframes, adhesion may be reduced due to a layer of copper oxide forming on the surfaces of the leadframe. The problems associated with this occurrence include delamination of the encapsulant, liquid trapped beneath the encapsulant during packaging and/or liquids seeping or vapors condensing under the encapsulant, leading to mold failure. These problems are addressed in various ways in the prior art. For example, the problems are described in U.S. Pat. No. 4,946,518 entitled “Method for Improving the Adhesion of a Plastic Encapsulant to Copper Containing Leadframes”. This patent teaches improving the adhesion of plastic encapsulants to copper leadframes by exposing the copper leadframes to an active oxygen ambient at temperatures below the leadframe annealing temperature. This strengthens any native copper oxide present on the leadframes without increasing the thickness of the leadframes.
In another example, in U.S. Pat. No. 5,459,103 entitled “Method of Forming Lead Frame with Strengthened Encapsulation Adhesion”, the leadframe is plated with copper strike and the copper strike is exposed to an oxidizing agent to form a layer of cupric oxide (CuO) that promotes adhesive bonding between the plastic mold and the leadframe.
The above approaches make use of exposed copper on the surface of the leadframe that is oxidizable to form copper oxide in the form of black oxide or brown oxide, which are good adhesion promoters for enhancing adhesion between the leadframe and the molding compound. However, there are certain types of leadframe widely used in the industry that have multiple layers of material plated onto the leadframe, such as pre-plated frames (“PPF”), so that it is not possible or feasible to have a layer of exposed copper that may be oxidized. One type of PPF leadframe is disclosed in European patent number 474,499 entitled “Lead Frame for a Semiconductor Device”. The leadframe described has a palladium or palladium-alloy film formed directly on the leadframe or indirectly on the leadframe via an underlying nickel layer. A silver or gold film is further formed on the palladium or palladium-alloy film. These additional layers of material are meant to enhance bondability and solderability of the leadframe surface.
A straight-forward way of manufacturing a PPF leadframe with plated nickel, palladium and gold layers would be to fully plate the leadframe with a layer of nickel film, followed by a layer of palladium film, and thereafter plating it with a layer of gold film. FIG. 1 is a schematic cross-sectional illustration of a PPF leadframe 100 of the prior art comprising full platings of nickel, palladium and gold on a copper-based substrate 110. The leadframe 100 comprises an external leads portion 102 and an internal leads portion 104. Located centrally of the internal leads portion 104 is a die pad 106 on which a semiconductor chip is to be mounted. The die pad 106 and inner leads portion 104 are located in an encapsulation portion 108 that is to be substantially encapsulated with molding compound during molding. The leadframe substrate 110 is fully plated with a layer of nickel 112, followed by a full plating of a layer of palladium 114 and thereafter, a full plating of a layer of gold 116. The problem with this manufacturing approach is that molding compound does not adhere well to nickel, palladium or gold. Without any special mechanism to promote its adhesion, adhesiveness is weak.
Therefore, another prior art construction tries to overcome this disadvantage by selective plating of the nickel, palladium and gold layers in order to expose the underlying copper-based substrate. FIG. 2 is a schematic cross-sectional illustration of a PPF leadframe 120 of the prior art comprising selective plating of nickel, palladium and gold on a copper-based substrate 110. The substrate 110 is selectively plated with a layer of nickel 112, so that some areas of the substrate 110 located within the encapsulation portion 108 are exposed. A layer of palladium 114 is then selectively plated over the layer of nickel 112 leaving the same areas of the substrate 110 exposed. Thereafter, a layer of gold 116 is similarly plated over the layer of palladium 114. The exposed copper in the areas of the substrate 110 that are not covered by plating can then be oxidized to form copper oxide to promote adhesion of the molding compound.
Although this approach is workable to improve adhesion, it is expensive because it involves multiple steps of mechanical masking to selectively plate the nickel, palladium and gold layers. Furthermore, this approach is difficult to implement on a large scale to mass production of such leadframes. An alternative approach of implementing selective plating with the application of photoresist instead of mechanical masking may improve speed and scalability, but incurs high costs.